1. Field of the Invention
The present invention relates to permanent magnets made of a 4f transition element-3d transition element alloy and, in particular, to a structure and a composition of a compound, which improves the magnetic properties of the permanent magnet.
2. Description of Related Art
An improvement in the performance of material for a permanent magnet can be indicated by three characteristics: Curie temperature, magnetization, and magnetic anisotropy. One known method for drastically improving these three characteristics is to insert a nonmagnetic atom to a parent-phase crystal of a magnetic compound. For example, as stated in JP-A 2008-78610, Sm2Fe17 (Sm: samarium, Fe: iron) is intruded by a non-magnetic element N (nitrogen) to improve the magnetic properties of the parent phase. In addition, as stated in academic paper 1 (Uebele et al.), when the non-magnetic element is F (fluorine), it is calculated that the most improvement in magnetic properties would be seen in R2Fe17 (R is a 4f transition element). Actually, as stated in academic paper 2 (Ardisson et al.), it has been known that the Curie temperature would be increased by the intrusion of the element F.
Academic paper 1: P. Uebele, K. Hummler, and M. Fahnle, “Full-potential linear-muffin-tin orbital calculations of the magnetic properties of rare-earth-transition-metal intermetallics. III. Gd2Fe17Z3 (Z═C, N, O, F)”, Phys. Rev. B 53, 3296 (1996).
Academic paper 2: J. D. Ardisson, A. I. C. Persiano, L. O. Ladeira, and F. A. Batista, “Magnetic improvement of R2Fe17 compounds due to the addition of fluorine”, J. Mat. Sci. Lett. 16 (1997) 1658.
Nd2Fe14B (Nd: neodymium, B: boron), which has the highest performance among existing permanent magnet materials, still requires a great amount of rare-earth element, which is a scarce resource (the atomic percentage of the element Nd to the element Fe is 14.3%). For this reason, it is important to use a composition having a smaller amount of rare-earth element than the above in improving magnetic properties. Sm2Fe17N3 described in JP-A 2008-78610 is improved in magnetic properties than its parent phase, but still has an insufficient magnetic moment and magnetic anisotropy. Gd2Fe17F3 reported by Uebele et al. is calculated to have an increased magnetic moment and an increased magnetic anisotropy; however, the stability of its crystal structure is not discussed so that whether it can stably exist as an actual system or not is unclear. The Curie temperature is not mentioned in Uebele et al. either. In the academic paper by Ardisson et al., no element analysis of F was performed with regard to R2Fe17Fx, thus, whether the effect is caused by the element F or not is unclear. In addition, it is challenged by the fact that the maximum increase in the Curie temperature is only about 40° C., which is still small.